This invention relates generally to controls for hydraulic power systems and more specifically to directional control valves that selectively effect automatic leveling of a bucket on the front end of a loader or similar device during movement of the boom arm to which the bucket is attached.
It is conventional practice to provide a hydraulic cylinder and separate control valve for manipulating the bucket of a front end loader and a second cylinder and companion valve for raising and lowering the boom of the loader. In the absence of any self-leveling function, it is necessary for the operator of the loader to operate both valves, one with each hand, to maintain the bucket level while raising the boom. This operation is not only difficult but also requires the strict attention of the operator. The advantages of self-leveling systems are obvious and there have been numerous types of systems on the market for many years.
One of the more common methods, such as shown in U.S. Pat. No. 3,987,920 to Parquet, is a mechanical linkage tied to the frame of the loader which tilts the bucket, maintaining it level as the boom is raised or lowered. Another common method, which is strictly hydraulic, is illustrated in U.S. Pat. No. 3,251,277 to Stacey. In this patent, the fluid displaced from the boom cylinder is directed to the bucket cylinder by actuation of the boom spool alone. This type of system requires a matching of volumes so that the volume displaced from the boom cylinder will extend the bucket cylinder the precise distance to hold the bucket level as the boom is raised.
Another self-leveling system is shown in U.S. Pat. No. 3,563,137 to Graber wherein the flow exiting the boom cylinder passes through a flow divider, dumping a portion to drain while directing the remaining portion to the bucket cylinder to maintain a level condition while raising the boom. In the last-mentioned patent, the excess oil is removed from the self-leveling circuit by a proportional flow divider dumping to drain. However, such a system can only be used in a conventional parallel circuit, as distinguished from a series type circuit to which the present invention is directed. In a series type valve, if the upstream valve is moved to an operative position, there is no pump pressure available to the remaining downstream valve since the power passages are in the series. Series type valves are normally not adaptable to a self-leveling function with the exception of the previously mentioned patent to Stacey; applicant's U.S. Pat. Nos. 4,561,342, 4,408,518 and applicant's pending U.S. application Ser. No. 464,071.
In the last-mentioned patent, a flow divider is utilized in the boom discharge flow path with a portion going to the bucket cylinder while the remaining flow passes back to the valve in the boom return motor port. This last-mentioned flow through the boom return port provides downstream oil for the bucket cylinder if the operator attempts to override the self-leveling function while the boom cylinder is moving.